Comment 10 for bug 1654600

Ok, here is how I understand the situation.

According to the systemd documentation about [Unit] Before= :

"Given two units with any ordering dependency between them, if one unit is shut down and the other is started up, the shutdown is ordered before the start-up. It doesn't matter if the ordering dependency is After= or Before=."

To me this means that network.service & local-fs.service will be shutdown _BEFORE_ unattended-upgrades-shutdown runs, hence /var var will be unmounted when it runs.

My current solution is to turn the unattended-upgrades.service ExecStart= into an ExecStop= so the unit will run as a shutdown instead of a start when the system shuts down :

[Unit]
Description=Unattended Upgrades Shutdown
DefaultDependencies=no
Before=shutdown.target reboot.target halt.target network.target local-fs.target
Documentation=man:unattended-upgrade(8)

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStop=/usr/share/unattended-upgrades/unattended-upgrade-shutdown --debug
TimeoutStopSec=900

[Install]
WantedBy=shutdown.target

Preliminary tests seem to run fine but I want to get confirmation on such a change by someone more expert with systemd that I am.